Airbags are typically mounted in housings within a steering wheel, or instrument panel, or other structure in the interior of a vehicle. The airbag is typically concealed behind a set of deployment doors and an airbag cover material. The interior mounting arrangement requires that the airbag cover be designed to satisfy both aesthetic and functional requirements. The airbag cover and tear seam should protect against fatigue, tampering and accidental damage and must satisfy the long term effects of weathering due to exposure to heat and sun. In addition, an aesthetic appearance may be a factor for the interior design of the vehicle. The airbag cover material is typically formed from plastic materials including thermoplastic materials.
Importantly, the airbag cover must be adapted to allow the airbag to deploy through the cover material. To this end, airbag covers typically include a tear seam that is of a reduced strength relative to the surrounding portions of the airbag cover. It is the tear seam that ruptures upon deployment of the airbag as the tear seam defines an area having a tensile strength that is lower than the surrounding portions of the airbag cover, and provides a weakened place in the airbag cover that allows a deploying airbag to break through. The tear seem may be formed during a molding process in producing the airbag cover, or by laser scoring as is known in the art. The tear seam may be visible as an indented line on the outside surface of the airbag cover, or may be hidden from view on the reverse side of the airbag cover so that it is not visible to the occupant of the vehicle.
Although the tensile strength of the tear seam is weaker than the remaining area of the airbag cover, the tearing of the tear seam is not easy to control under deployment conditions. Neither the rate of tearing of the tear seam nor the location of the initial tear are easily controllable. As a result, efforts to develop airbag covers continue, and a variety of attempts have been made to develop a tear seam configuration that will support uniform deployment of the airbag.